But, the Arduboy doesn’t run with a 5V VCC (even when USB powered). It runs directly off of the li-ion battery voltage of 4.2V max (~3.7V nominal and ~3V min).
So, using a schottky diode (i.e. 1N5818) with a voltage drop of around 0.3-0.4V, the flash IC will see a VCC of around 3.8-3.9V max, which is closer to it’s normal operating range max of 3.6V. Without the diode, the 4.2V max battery voltage is still under the 4.6V absolute maximum value, but getting closer to the 3.6V range is probably worth it.
Actually, using a standard silicon diode (i.e. 1N4148) may work as well, because at the low current the flash IC will see, it’s voltage drop is only about 0.4-0.5V as well. But, this may put the input high levels too close to that VCC+0.4V rating.